Oh, I'm reciting this mantra, but mainly without counting, as I'm neither a monk or nun nor in retreat, so I often recite "on the way".
Cundi is really a great teacher, it's good to make contact with him/her (as I'd agree he's more male than female).
A sadhana can be found at Bill Bodris meditationexpert homepage (or maybe even via google..?).
I've had several wonderful experiences, including a lot of help in leaving bad ways of spirituality, finishing things I wasn't able to finish before, and a general progress in my practice. Concerning concrete "application" I also got a lot of help with a really, really bad injury on a non-material level. But I guess I'm still far away from having recited it even 200,000 times (mantra recitation is not my only practice, and actually in the beginning my practice had been completely different from anything like this), so I'm sorry, but can't tell you about effects of more than this. If you have the time to do one of these rituals reciting it so often, I'd be curious for your experiences
I experience that it's helping a lot for purification, at least purification of your body, speech and mind; I think that IS what you would call removing bad karma. That, in turn, also improves the efficacy of other practices; so I can really recommend reciting this mantra.
He's a very soft teacher, though it is difficult to discribe how this way of "leading" and "teaching" works if you haven't experienced it yourself, and it might also depend on your self-reflective capabilities and your supernatural perception whether you notice that at all. Actually, though I have some lower visions and minor stuff like that, I'm FAR away from clearly perceiving him/her as s/he really is. So of course I've not completely proven it, but concerning the things I already tried and experienced they always worked, so as far as my limited experience allows a judgement I would say that every single word of this dharani is true
It can really bring you out of hell, if necessary.
But some words about visions: If you'd ask me, I for my part don't care so much about experiencing something like that. I frequently have the "lower" kind of visions you can have as long as your third eye is not completely open. Somehow they are not so much more but a kind of perceptions, or in other words, one of our deluded ways to perceive things, so the "vision" itself is not the whole truth. How could a vision, as the simulation of the perception of your eyes, be the "whole" truth anyway? .. and I think, the whole truth is what you are or should really be after once you've taken refuge to the Buddha. With the aim to help other beings of course, by realizing the truth; but you really shouldn't be too fond of perceptions.
By the way, reciting the heart sutra is a good additional practice. And being kind to others and improving your virtues, which might mean that sometimes people will hate you for being virtuous. But you can mantra as much as you want - what's the worth of it if it wouldn't help you to in turn help others on their way towards enlightenment?
If you ask me, honestly - I think if you really do 300,000 repetitions or more, I think nothing's impossible with this mantra, so I don't think it's a metaphor, though it might well be that until you reach this number of repetition your aims, perceptions, and your view of the world in general might have changed a lot
Have a good evening (ehm.. at least for me it's evening..)